1. Field of the Invention
The present invention relates to a process for manufacturing MgO-based .beta."-alumina sintered bodies, specifically, such sintered bodies with high density and ionic conductivity.
2. Related Art Statement
The MgO-based .beta."-alumina sintered bodies are ceramics to be used as a solid electrolyte, for example, for sodium-sulfur cells or the like. These sintered bodies have hitherto been manufactured by shaping a mixed starting powder comprising Al.sub.2 O.sub.3, Na.sub.2 CO.sub.3, MgO and the like components or, .beta.- or .beta."-alumina powder previously synthesized by firing such a mixed starting powder, and then firing the resulting shaped bodies.
Namely, Japanese Patent Publication No. 8842/'77 discloses a manufacturing process of .beta."-alumina sintered bodies wherein Al.sub.2 O.sub.3, sodium aluminate and MgO are mixed together, shaped and then fired. Alternatively, in Japanese Journal of Applied Physics, Vol. 11, No. 2, pp. 180-185 (1972) is described a process wherein .beta.-alumina powder is admixed with Na.sub.2 CO.sub.3 and MgO and provisionally fired to provide a .beta."-alumina-.beta.-alumina mixed powder which is then shaped and fired. Furthermore, in Solid State Ionics, Vol. 13, 53-61 (1984) is described a process wherein Al.sub.2 O.sub.3, Na.sub.2 CO.sub.3 and an MgO-Al.sub.2 O.sub.3 reaction product are mixed together and provisionally fired to provide a .beta."-alumina-.beta.-alumina mixed powder which is then shaped and fired.
As shown in Solid State Ionics, Vol.13, 53-61 (1984) and also in Silicates Industries, 81-85 (1986), the MgO-based .beta."-alumina, when it has a composition close to 87 wt. % Al.sub.2 O.sub.3 4.5 wt. % MgO and 8.5 wt. % Na.sub.2 O, can be provided with a .beta."-alumina phase having a high ionic conductivity. However, according to conventional processes, a difficulty has been encountered in the manufacture of .beta."-alumina sintered bodies of such a high conductive composition, because sintering is hardly effected due to grain growth occurring during the course of manufacture.